How to Composite a “Front-Light / Back-Light” Shot In Adobe After Effects CS6
A few years ago, my friend Drew Christie re-introduced me to the work of John Taylor, a folk artist and sculptor from Orange County, California. I instantly realized I’d seen his work before at the Henry Art Gallery in Seattle when I was much younger, and I remembered being enthralled by his sculptures of historic ghost ships, whales and old vehicles pieced together from scraps of wood, found objects, old computer parts and random junk.
When I started working on my Thesis film at CalArts, I began thinking about John’s ships a lot. I remember looking them up online for inspiration and when I saw that he also lives in Southern California, I got up the courage to email him directly. To my surprise he responded promptly with an extremely friendly email and he invited me to his home where I was able to see his workshop in person.
Later on I began to imagine the movie opening with shots of a ghost ship like John’s, but I wasn’t quite sure how I’d go about compositing something like that into the After Effects comps I’d built out of live-action footage and photo-collage. To cut a long story short, I ended up taking another trip to visit John and to my excitement, he offered to lend me his sculpture of the Charles B. Hill, a Detroit freighter from 1906. After that, all I had to do was figure out how to shoot it and composite into my shots.
I remembered learning about a technique for shooting stop-motion in Helder K. Sun’s Cinematography and Lighting Class at CalArts called “front-light / back-light,” which creates amazing mattes for compositing models and puppets. However, Helder never taught us how to composite the footage once we’d shot it. A few weeks ago, my friend Quique Rivera figured out how to work with “front-light / back-light” footage in After Effects. After he taught me I started experimenting heavily with the technique, and I thought I’d do a blog post illustrating the process I used to composite together this shot I animated using John Taylor’s sculpture of the Charles’ B. Hill:
I made this shot on a Canon 5D Mark II with a 100mm Canon L-Series Macro Lens. I captured all the images frame by frame in Large Raw + Large Fine JPEG using Dragon Frame, a Stop-Motion software designed and developed by Jamie Caliri, a CalArts alumni. I then composited everything in Adobe After Effects CS6. Both front-light and back-light exposures were set to 320 ISO with 4s shutter and an f-stop of 32. It’s true the exposure time was painfully long, even with two 2×2 Kino-Flos placed relatively close to the ship and the screen. But, this way I was able to achieve a deep focus at a 320 ISO, which is supposedly the sweet spot for the Canon 5D Mark II. If you’re curious, here’s how I set up the shot in After Effects:
1.) This is a still from the front lit footage. As you can see, the white board looks grey when the back-light is off. Having said that, the wheelhouse windows are bright white because I rigged two small LED lights on a piece of armature wire to shine through them. That way the back-light matte will make the windows semi-transparent allowing me to easily composite a character inside the cockpit later. This is something I tested out in After Effects first before making the decision to shoot it that way.
2.) Here’s the same still from the back-light footage. I added heavy levels and curves to boost the contrast and kill any gray colors on the white board.
3.) Once you’ve placed the back-light footage above the front-light footage, all you have to do is choose Luma Inverted Matte, under the TrikMat drop down menu and “voila!” you have a perfect matte that moves along with your footage and doesn’t wiggle around the edges like keyed green screen or blue screen footage.
4.) Finally, because I animated the camera movement by hand, I used Warp Stabilizer in After Effects to smooth out some of the jiggling motion. Also, because I shot this on a 100mm Macro Canon L-Series lens on a Canon 5D Mark II body there was a slight bit of flicker due to the mechanical closing of the aperture for each photograph. This could have been avoided by unscrewing the lens half way to disengage the connection to the camera body. But, because the 100mm Macro is physically quite long, I was nervous about it falling off and smashing on the floor so I decided to fix the flicker in post using Auto Levels in After Effects. I also tested this prior to shooting and all I had to do was readjust the curves to regain the original look.
It’s a pity I didn’t take any production stills, but I finished animating this shot at 6AM and I had no energy or cell phone battery left to take photographs of the setup. As a result, a description will have to do. The entire shot was made with a geared tripod head screwed onto 10 foot animation slider track. However, I only used 6 inches of it and I marked out my animation increments on a piece of paper tape stuck along the track using a miniature architects ruler. Then, I lined up part of the model with the edge of the frame to animate the pan of the geared tripod head. As you can see with a little Warp Stabilizer and some After Effects work I was able to achieve something almost as clean as a motion control shot. Therefore, I feel like this could be interesting for anyone wanting to make inexpensive but smooth camera movements in stop-motion in order to take advantage of frame by frame techniques like “front-light / back-light.”
Of course, the down side to using this technique is you have to manually animate everything frame by frame and the camera movements have to be extremely precise. The ideal way to do “front-light / back-light” is to use a Motion Control Rig with a Chauvet light controller box hooked up to a DMX box hooked up to Dragon Frame. Then you program Dragon Frame to automate the lights and the exposures. Most likely, very few people who are reading this will have access to any of these things let alone Motion Control.
However, the Chauvet box at CalArts has been broken for quite some time so I’ve never been able to automate the lights with the DMX box. Therefore, I’ve always manually switched them off and on, which is time consuming, but it works fine. I have used both Motion Control and manual animation for these kind of camera moves and both work fine. MOCO is much faster and the movement much more precise, but renting a MOCO stage outside of CalArts would be extremely expensive and Kuper, the MS-DOS software that runs most MOCO rigs, has a pretty steep learning curve.
To set up a “front-light / back-light” shot you first light your model so it matches the scene you’d like to composite it into. Then you blast a diffused light on a white reflector board behind the model so that it’s completely white diffused and devoid of hot-spots. Then you animate the camera and set up Dragon Frame to take two different exposures, one with the front-light turned on and the back-light turned off, and one with the front-light turned off and the back-light turned on. As I said earlier, if you don’t have a DMX box and a Chauvet you’ll have to manually turn the lights on and off yourself. I suggest setting up two multi-taps / power strips right next to you and flipping the switch each time. This is method is much safer because you won’t risk bumping your set by running around and switching on and off your lights.
The trick to getting the lighting right is to make it so there’s no spill from the white board on the model. But, this is actually pretty forgiving because you can push the contrast of the back-light shot later on in After Effects. As long as the black parts are black, you won’t get transparencies in the rest of the model. If the lighting is decent and you composite these two pieces of footage using the Luma Inverted Track Matte in After Effects, the result will be much better than any green screen or blue screen key you’ve ever pulled.
Actually, I’ve found that although this whole process is extremely tedious, it’s much less time-consuming and more enjoyable on the post production end, because the matte is so perfect and instantaneous. Even if you don’t plan on moving the camera, I feel like the setup is almost worth it for a single photograph. “Front-light / back-light” will especially work wonders for a matte of something extremely complex like hair or fur. After all, the thought of cutting out a single image of complex hair in Photoshop gives me a headache.
Stayed tuned and I’ll upload more “front-light / back-light” shots and production stills from future shoots.